The present invention relates to an improved process for preparing halogen-free, reactive polyisobutene having a terminal double bond content of more than 60 mol % and an average molecular weight Mn of 800-3000 dalton by cationic polymerization in the liquid phase of isobutene over an acidic, essentially halogen-free heterogeneous catalyst, the increased terminal double bond content being obtained by partial conversion.
The polymerization of isobutene yields an inseparable mixture of polyisobutenes, in which the position of the double bond varies between the individual polyisobutenes. Polyisobutenes of formula I 
where n is the degree of polymerization which in turn is derived from the average molecular weight Mn of the polyisobutene prepared, contain terminal Cxe2x80x94C double bonds of the vinylidene type which are herein also referred to as xcex1-olefinic double bonds owing to their position in the polyisobutene molecule. Accordingly, the double bonds in polyisobutenes of the formula II 
are referred to as xcex2-olefinic. If the polymerization of isobutene is carried out without taking special measures, a random mixture is formed which comprises polyisobutenes having xcex1-olefinic, i.e. terminal, double bonds, xcex2-olefinic double bonds and double bonds located further toward the interior of the polyisobutene molecule. Both the terminal double bond content and the xcex2-olefinic double bond content of a polyisobutene product prepared by a particular process are both reported in mol %.
The preparation of polyisobutenes is described, for example, in H. Gxc3xcterbock, Polyisobutylene und Mischpolymerisate, p. 77-104, Springer-Verlag, Berlin, 1959. They are usually prepared by Lewis-acid-catalyzed isobutene polymerization employing aluminum chloride, alkylaluminum chloride or boron trifluoride as Lewis acids. However, the resulting polymers have a relatively low vinylidene-type terminal Cxe2x80x94C double bond content of less than 10 mol %.
In contrast, reactive polyisobutene (PIB) having molecular weights of usually 500-5000 dalton has a high terminal vinylidene group content of, preferably, more than 50 mol %. These reactive polyisobutenes are used as intermediates in the preparation of additives for lubricants and fuels as described, for example, in DE-A 27 02 604. These additives are prepared by initially reacting polyisobutene with maleic anhydride. The preferred reactive sites for this reaction are terminal double bonds of the vinylidene type, whereas double bonds located further toward the interior of the macromolecule react to a lesser extent, if at all, depending on their position in the molecule. The polyisobutene/maleic anhydride adducts formed are then reacted with certain amines to give the corresponding additives. It is therefore absolutely necessary for polyisobutenes used as starting materials for the abovementioned additives to have a high terminal double bond content. The same applies to the preparation of the polyisobuteneamines of EP-A 244 616 which are also used as fuel additives and which are prepared by hydroformylation of the reactive polyisobutene and subsequent reductive amination of the resulting polyisobutene aldehyde. For this process, preference is likewise given to using polyisobutene having a high terminal double bond content, but xcex2-olefinc polyisobutenes also give the desired product when the hydroformylation is carried out using cobalt catalysts, owing to their double bond isomerization activity.
Most prior art methods for the preparation of reactive polyisobutene involve the homogeneously catalytic polymerization of isobutene. According to DE-A 27 02 604, for example, a polyisobutene product having a terminal double bond content of up to 88% is obtained by reacting isobutene in the presence of boron trifluoride. EP-A 145 235 teaches the polymerization of isobutene in the presence of a complex of boron trifluoride and a primary alcohol at from xe2x88x92100xc2x0 C. to +50xc2x0 C. to give products having similarly high vinylidene double bond contents. According to U.S. Pat. No. 5,286,823, highly reactive polyisobutene can also be prepared using complexes of boron trifluoride and secondary alcohols as catalyst.
The disadvantages of this homogeneously catalyzed process are that the Lewis acid catalysts used are corrosive and that there is a risk that, in addition to the desired reactive polyisobutene, halogenated polymeric byproducts are formed which are virtually inseparable from PIB and adversely affect the product and further processing characteristics of the PIB. In these processes, the homogeneous catalyst is usually separated off by quenching with a nucleophile to destroy the catalyst and subsequently removing the PIB from the quenching mixture by extraction. These additional work-up steps are a further disadvantage of the homogeneously catalyzed process.
Furthermore, WO 94/28036 discloses the preparation of polyisobutene using heterogeneous Lewis acid-like catalysts. Catalysts used are salts of elements of transition groups III, IV, V and VI of the Periodic Table of the Elements which are insoluble in the reaction medium, preferably halides, sulfates, perchlorates, trifluoromethanesulfonates, nitrates and fluorosulfonates thereof. The polymerization is terminated by adding methanolic ammonia solution to the reaction medium to destroy or at least substantially inactivate the catalysts in question.
The preparation of PIB using heterogeneous, non-salt like catalysts is also known. For example, U.S. Pat. No. 4,288,649 describes a process for producing polyisobutene having an average molecular weight  greater than 1250 dalton by polymerizing C4-hydrocarbon mixtures comprising isobutene over halided alumina catalysts. The catalysts are prepared by treating the alumina with a haliding agent, preferably with a chloriding agent, in particular with carbon tetrachloride, at an elevated temperature. A disadvantage of this process is that some of the chlorine is transferred from the catalyst to the polymer which forms. For example, the polymerization of a mixture of n-butane, isobutane and isobutene over a chlorided alumina catalyst prepared in this manner gives, after a reaction time of 2 hours, a polyisobutene product having a chlorine content of 46 ppm.
U.S. Pat. No. 5,326,920 discloses a process for polymerizing isobutene employing as heterogeneous catalyst an oxidic support material, preferably silica, which has been activated with a metal chloride attached thereto, preferably with an aluminum chloride. Particular preference is given therein to an SiO2-AlCl2 catalyst in which AlCl2 groups are anchored on the SiO2 support via oxygen linkages. The disadvantages of this process are that the polyisobutene products obtained have an extremely broad molecular weight distribution D of from 8 to 14, a low terminal double bond content and a chlorine content in the ppm range. Furthermore, this process requires the presence of promoters such as water, alcohols, alkyl halides or hydrogen chloride to achieve a catalyst activity which is sufficient for industrial operation. Similar catalyst systems for the polymerization of isobutene are described in WO 95/26815, WO 95/26816, WO 95/26814 and WO 96/26818.
JP-A 139 429/1981 utilizes heterogeneous zirconium dioxide and molybdenum oxide catalysts to prepare isobutene oligomers having a molecular weight of less than 300 dalton. These catalysts can be mixed with aluminum fluoride to increase their activity. According to this publication, for example, the reaction of an isobutene-comprising C4 cut (composition: 46% of isobutene, 28% of 1-butene, 8% of 2-butenes, 12% of n-butane, 5% of isobutane, 1% of 1,3-butadiene) over an MoO3xe2x80x94ZrO2 catalyst having a molybdenum content, calculated as MoO3, of 13% by weight at 120xc2x0 C. yields an isobutene oligomer mixture comprising 29% of diisobutene, 49% of triisobutene and 19% of tetraisobutene.
EP-A 825 205 describes, inter alia, the continuous polymerization of pure isobutene over heterogeneous catalysts (heteropolyacids), but without giving any details about a specific reaction procedure to obtain polyisobutene having a high terminal double bond content, although its preparation is an object of said European patent application. However, the choice of the heteropolyacids as catalysts has proved to be insufficient to achieve this object.
DE-A 19 52 8942 describes the preparation of low molecular weight, reactive and halogen-free polyisobutene utilizing particular catalysts comprising a zirconium oxide support material, where it is also possible to carry out the polymerization using hydrocarbon mixtures comprising isobutene as a continuous process. It is claimed that the average molecular weight is controlled by varying the space velocity and residence time in the continuous polymerization process. However, the examples only described batchwise polymerization processes. In Examples 11-16, in which a hydrocarbon mixture comprising isobutene was used, some of the reactions were performed at low conversions and gave some high and some low selectivities for polyisobutene having an xcex1-double bond. The use of other catalysts gave similar results according to the prior German applications DE-A 1 97 04 482 and DE-A 1 97 34 176.
EP-A 825 205 discloses a process for the polymerization of olefins in the presence of heteropolyacids. The products include polyisobutene having a high content of terminal vinylidene units. It is an object of the present invention to provide a process which makes it possible to prepare low molecular weight polyisobutene having a high a-double bond content continuously and reproducibly over a heterogeneous catalyst.
We have found that this object is achieved according to the invention by a process for preparing halogen-free, reactive polyisobutene having a terminal double bond content of more than 60 mol % and an average molecular weight Mn of 800-3000 dalton by cationic polymerization in the liquid phase of isobutene over an acidic, essentially halogen-free heterogeneous catalyst, in which:
a) a hydrocarbon mixture of essentially C4-hydrocarbons comprising isobutene in an amount of from 10 to 80% by weight is used as the starting material and
b) polymerization is carried out continuously at from xe2x88x9230 to 0xc2x0 C. with average starting material residence times of one hour or less, where the temperature and the residence time are selected such that the isobutene conversion is less than 60% and, after separation from the resulting polyisobutene, the isobutene is either enriched in the partially converted hydrocarbon mixture and returned to the polymerization or passed to another isobutene reaction coupled with the polymerization.
Catalysts which may be used according to the invention are in principle any acidic, essentially halogen-free catalysts capable of catalyzing isobutene polymerization at the above-described temperatures. Preference is given to catalysts based on zirconium dioxide as described in DE-A 19 52 8942, supported W and Mo oxides as described in DE-A 1 97 04 482, and heteropolyacids or salts thereof as described in DE-A 1 97 34 176 and EP-A 925 205. Particular preference is given to zirconium dioxide systems which comprise PO4 and/or SO4 and are furthermore doped with oxides of transition metals such as Mn, Fe (DE-A 19 52 8942), or salts of dehydrated Mo or W heteropolyacids comprising Cs.
Prior to use, the catalysts to be employed according to the invention are advantageously shaped to give shaped articles such as tablets, spheres, cylinders, rings or spirals or comminuted to give granules in a conventional manner and preferably used in this form in a fixed bed in the reactor or milled to give a powder and used in this form, for example as suspension catalysts.
The abovementioned patents essentially describe the following catalysts:
A) zirconium dioxide systems doped or promoted with elements of transition groups I-VIII or main groups II-VI of the Periodic Table of the Elements, in each case in the form of oxygen-containing compounds. Preferred promoters of transition groups of the Periodic Table of the Elements are oxygen-containing compounds of the elements Cu, Ag, Zn, Sc, Y, La, Ce, Nd, Pr, Ti, Hf, V, Nb, Ta, Mo, W, Re, Co and Ni and in particular of Mn and Fe. Preferred promoters of the main groups of the Periodic Table of the Elements are oxygen-containing compounds of the elements Ca, Mg, B, Ga, Si and Sn and in particular of the elements Al, S and P. Examples of preferred combinations of promoting elements are the combinations of S and/or P with Fe, Fe/Mn, Mn or Fe/Al. The combination of S and/or P with Mn is most preferred.
B) Heterogeneous polymerization catalysts consisting of oxides of the elements of transition groups V and VI of the Periodic Table of the Elements (unsupported catalysts) or supported oxides of the elements of transition groups V and VI of the Periodic Table of the Elements. In one embodiment, the elements of transition groups V and VI of the Periodic Table of the Elements are supported on a non-zeolitic oxidic support which is not an oxygen-containing zirconium compound. Preferred oxides of the unsupported catalysts and preferred promoters of the supported oxides are oxides of the elements V, Cr, Mo and W. Particular preference is given to oxides of molybdenum. Particularly useful support materials include the oxides of the elements B, Pb, La and especially the oxides of the elements A1, Si, Fe and Ti.
C) Heteropolyacids (in the form of unsupported or supported catalysts): preference is given to heteropolyacids having, as the xe2x80x9cpolyatomxe2x80x9d, tungsten or molybdenum, some of which may be replaced by vanadium, and having, as the central atom, silicon or preferably phosphorus. Examples of heteropolyacids comprising molybdenum in the form of their free acids are: dodecamolybdophosphoric acid (H3PMo12O40*xH2O); dodecamolybdosilicic acid (H4SiMo12O40*xH2O), dodecatungstophosphoric acid (H3PW12O40*xH2O) and dodecatungstosilicic acid (H4SiW12O40*xH2O). Preferred support materials are: activated carbon, SiO2, TiO2, ZrO2.
D) Salts of the heteropolyacids (in the form of unsupported or supported catalysts): in the heteropolyacid salts, some or all of the hydrogen ions (protons) of the free heteropolyacids are replaced by other ions, e.g. metal ions or ammonium ions. Particularly useful systems are those in which the protons of the HPAs comprising tungsten or preferably molybdenum are replaced by ions of the elements Mn, Fe, Co, Ni, Cu, Ag, Zn, Na, K, Cs, Mg, Ca, Ba, B, Al, Bi or by ammonium ions (NH4+) or alkyl-substituted ammonium ions (NR4+). Particular preference is given to cesium salts of heteropolyacids (xe2x80x9cHPAsxe2x80x9d) comprising tungsten and especially HPAs comprising molybdenum. Typical support materials are activated carbon, SiO2, TiO2 or ZrO2.
The catalysts to be used according to the invention can be stored over a virtually unlimited period of time, in particular with exclusion of moisture. Catalysts which have become moist are advantageously dried under atmospheric pressure or reduced pressure, under atmospheric pressure at from 180 to 250xc2x0 C. under reduced pressure also at lower temperatures, prior to use.
Typical feed materials are hydrocarbon mixtures of essentially C4-hydrocarbons comprising isobutene which are obtained in large amounts e.g. in hydrocarbon cracking processes. For example, thermocracking processes (streamcracker) produce C4 cuts comprising isobutene from which the butadiene is substantially removed by additional selective hydrogenation or extractive distillation using N-methylpyrrolidone. The resulting xe2x80x9craffinate Ixe2x80x9d is suitable for PIB synthesis and has essentially the following typical composition: 44-49% of isobutene, 24-28% of 1-butene, 19-21% of 2-butene, 6-8% of n-butane, 2-3% of isobutane (Weissermel, Arpe, xe2x80x9cIndustrielle Organische Chemiexe2x80x9d, Verlag Chemie, Weinheim, 1994, p. 76).
Alternatively, it is possible to use C4 streams comprising isobutene obtained by catalytic cracking in refinery processes. This C4 FCC cut (Fluid Catalytic Cracking) has the following typical composition: 15% of isobutene, 12% of 1-butene, 23% of 2-butene, 13% of n-butane, 37% of isobutane.
However, particular preference is given to using isobutene/isobutane mixtures which can be obtained from n-butane by isomerization, e.g. according to the Butamer(copyright) process (UOP), and subsequent dehydrogenation, e.g. according to the Catofin(copyright) process (Weissermel, Arpe, xe2x80x9cIndustrielle organische Chemiexe2x80x9d, Verlag Chemie, Weinheim, 1994, p. 79, 80). The dehydrogenation of isobutane according to the Catofin(copyright) process (ABB Lummus Crest) typically yields an isobutene selectivity of 92-93 mol % at an isobutane conversion of 65%. Alternative processes such as Oleflex(copyright) (UOP), STAR (Phillips), FDP (Snamprogetti/Yarsintez) operate at lower isobutane conversions (45-55%) and similarly high isobutene selectivities (87-92 mol %) (PERP report butadiene/butylenes 92-1, 1993, CHEM SYSTEMS, p. 32).
It is preferred to use isobutene-isobutane mixtures as feed materials for the synthesis of polyisobutene since the polymerization according to the invention only converts some of the isobutene and produces a halogen-free residual stream which consists virtually exclusively of C4-hydrocarbons and which may be separated from the polyisobutene by distillation and used for a variety of purposes, for example:
Separation of the residual C4 stream (isobutane (bp. xe2x88x9212xc2x0 C.)/isobutene (bp. xe2x88x927xc2x0 C.)) by distillation into isobutane (overhead product, not necessarily free of isobutene) and a mixture of isobutane and isobutene (bottom product) equivalent to the composition of the feed and subsequent return of the bottom product to the polymerization and return of the isobutane to the dehydrogenation (preferred embodiment).
Return of the residual C4 stream to the dehydrogenation
Utilization of the residual C4 stream for the synthesis of methyl tert-butyl ester (MTBE).
The polymerization is carried out continuously at generally from xe2x88x9230 to 0xc2x0 C. preferably from xe2x88x9225 to xe2x88x9210xc2x0 C. under atmospheric pressure or superatmospheric pressure, especially under the autogeneous pressure of the reaction system, so that the hydrocarbon mixture comprising isobutene remains in liquid form.
Surprisingly, it has been found that when an isobutene partial conversion (up to 60%, particularly preferably up to 50% and especially up to 40%) was achieved by appropriate choice of the reaction parameters, the xcex1-olefin contents of the PIB products were always particularly high (i.e. regularly xcex1 greater than 60 mol %, preferably  greater than 65 mol % and especially  greater than 68 mol %).
It is possible to use conventional reactors such as loop reactors or reactor batteries in the continuous polymerization according to the invention. It is also possible to use tubular reactors or tubular reactor batteries operated in upflow or downflow mode. It is advantageous for the catalysts to be used according to the invention, preferably when using loop reactors or tubular reactors, to be arranged in a fixed bed or to be suspended in the reactor medium in powder form. Typical space velocities are from 10 kg isobutene/kg cat. *h to 0.1 kg/kg*h, preferably from 7 to 0.2 kg/kg*h, particularly preferably from 5 to 0.32 kg/kg*h. Depending on the design, typical residence times are from 0.05 to 1 h, preferably from 0.1 to 0.9 h, particularly preferably from 0.2 to 0.8 h.
The isobutene polymerization can be carried out with or without preferably apolar, halogen-free solvents, preferably hydrocarbons. Specific examples of these are butanes, pentanes, hexanes, heptanes and octanes which can be linear or branched. Particular preference is given to butanes, pentanes and hexanes. Because of the exothermic nature of the isobutene polymerization it may be advantageous to provide the reactors used with internal or external cooling means.
The desired average molecular weight Mn of the polyisobutene can be adjusted by variation of the reaction parameters, in particular by variation of the temperature, residence time, amount of solvent and space velocity. The residence time is generally from 0.01 to 1 hour, preferably from 0.1 to 0.8 hour. The molecular weights Mn are in the range from 800 to 3000, preferably from 1000 to 2300.
The isolation of the polyisobutene from the polymerization mixture generally does not include any special technical features and may be effected by distillation, which, if a suspended catalyst is used, is preceded by the removal of the suspended catalyst, for example by filtration, centrifugation or decanting. The distillation advantageously initially removes volatile constituents of the polymerization mixture such as unconverted isobutene, hydrocarbons present in the starting material or added as solvents and then higher-boiling byproducts, for example low molecular weight isobutene oligomers from the polyisobutene.
The process of the invention provides a particularly economical way to prepare reactive, halogen-free polyisobutene having an average molecular weight Mn of generally 800-3000 dalton and a terminal double bond content of more than 60 mol %.